Ako bol ne prođe, zvat ćemo hitnu.

Ako means if.

The sentence has two parts:

• Ako bol ne prođe = If the pain doesn’t go away
• zvat ćemo hitnu = we’ll call an ambulance / emergency medical help

So the structure is a normal if + result pattern:

Ako X, Y = If X, Y

Here, the if-clause comes first, so it sets the condition for what will happen next.

This is normal in Croatian.

After ako (if), Croatian often uses the present tense to talk about a possible future situation. English does something very similar:

• If the pain doesn’t go away, we’ll call an ambulance.

We do not normally say:

• If the pain won’t go away, we’ll call an ambulance in this kind of neutral prediction.

So Ako bol ne prođe is literally in a present form, but it refers to a future possibility: if the pain doesn’t pass/go away.

This is mainly about aspect.

• prođe comes from proći, a perfective verb
• prolazi is an imperfective form

In this sentence, ne prođe suggests the pain fails to pass/go away completely. That fits well with a one-time future condition.

A rough contrast:

• Ako bol ne prođe = If the pain doesn’t go away
• Ako bol ne prolazi = If the pain isn’t going away / keeps not getting better

Both can make sense, but ne prođe is very natural when talking about whether something will resolve or not.

In Croatian, ne is usually a separate negative particle placed before the verb:

• ne znam = I don’t know
• ne mogu = I can’t
• ne prođe = doesn’t go away

So ne prođe is the normal spelling.

There are some common exceptions where the negative form is written together, such as:

• nemam
• nisam
• neću

But with prođe, you write ne separately.

Because Croatian often leaves out subject pronouns when they are already clear from the verb form.

ćemo shows 1st person plural, so zvat ćemo already means we will call.

You could add mi for emphasis:

• Mi ćemo zvati hitnu = We will call the ambulance

But in ordinary speech, the pronoun is usually unnecessary.

Yes. Both are correct ways to express the future.

Croatian future tense is formed with:

• the infinitive
• plus forms of htjeti (ću, ćeš, će, ćemo, ćete, će)

With zvati:

• zvat ćemo
• mi ćemo zvati

Both mean we will call.

A useful point: when the infinitive comes before the auxiliary, the final -i of zvati is dropped in standard writing:

• zvat ćemo ✅
• zvati ćemo ❌ standardly avoided

Also, ćemo is a clitic, so it normally does not stand at the very beginning of the clause by itself. That is why you get either:

• Zvat ćemo hitnu
• Mi ćemo zvati hitnu
• Onda ćemo zvati hitnu

but not normally just Ćemo zvati hitnu.

Here hitnu is short for hitnu pomoć.

• hitna pomoć = emergency medical help
• in everyday usage, hitna / hitnu often means the ambulance or emergency medical services

So:

• zvati hitnu means
• call an ambulance
• call emergency medical help

This shortening is very common in Croatian.

Because it is the direct object of the verb zvati (to call), so it must be in the accusative case.

The full phrase is:

• hitna pomoć = nominative
• hitnu pomoć = accusative

Since pomoć is omitted, the adjective stays in the same case:

• zvati hitnu (pomoć)

So hitnu is the correct form here.

Bol is in the nominative singular because it is the subject of prođe.

So in:

• Ako bol ne prođe

bol = the pain / pain as the thing that may or may not go away.

Croatian does not have articles like a or the, so context tells you whether the meaning is:

• pain
• the pain
• sometimes a pain

In this sentence, English will usually translate it as the pain, but Croatian simply says bol.

A useful extra note: bol is a feminine noun, even though it ends in a consonant.